Referring to FIG. 7A, there is shown a rough sketch of conventional optical aperture expansion using diffractive components in a waveguide. In the current figure, incoming light (image) is vertical from outside the page into the page. Coupling-in element 1001 couples the incoming light into lateral expansion element 1002, which expands the light laterally (from left to right in the current figure). Then the laterally expanded light is coupled into vertical expansion element 1003 which expands the light vertically (from top to bottom in the current figure), and couples-out the light to a user (eye of a viewer).
Conventional diffractive elements introduce chromatic dispersion where light-rays with different wavelengths diffract at different angles. To reduce chromatic dispersion narrowband light sources (such as lasers) can be used. A more practical solution is to design the diffractive components to cancel the dispersion of each other.
Referring to FIG. 7B, there is shown a diagram of the FIG. 7A diffraction directions of light propagating in the angular domain (angular space). Dashed arrows and solid arrows show two different exemplary wavelengths. The starting angle at area 1005 represents the angle of the light rays as the light rays impinge on the first diffractive element (coupling-in element 1001) to be coupled into the lightguide. Area 1007 represents the direction of the light rays after coupling in element 1001, Area 1009 after lateral expansion element 1002, and the area 1005 also represents the angle of the light rays after coupling out of the lightguide by vertical expansion element 1003. The direction of the light rays entering the lightguide is equal to the direction of the light rays coupled out of the lightguide in order to minimize chromatic dispersion. It is apparent that different wavelengths will have different directions as the different wavelengths of light propagate within the lightguide, and will have the same direction when output from the lightguide.